1. Field of the Invention
The present invention relates to styrene/maleic anhydride copolymers which are formulated to obtain compositions of reduced combustibility and which evolve less smoke during combustion.
2. Description of the Prior Art
Styrene is a term used herein to describe vinylaromatic monomers generally such as styrene itself as well as ring and side chain substituted derivatives like alpha-methylstyrene and/or chlorostyrene.
The preferred styrene/maleic anhydride polymers used in the invention are those in which the styrene is polymerized with from 5 to 30 percent by weight of the copolymerized monomers of maleic anhydride.
Such styrene/maleic anhydride (S/MA) copolymers are conventionally compounded with a halogen-containing component to reduce their flammability. At the combustion temperature of the polymer, this component releases halogen and inhibits the spread of any flame by a vapor-phase free-radical mechanism.
However, this result can also be achieved by cooling down the pyrolysis zone through the incorporation of an additive which decomposes with absorption of large amounts of heat with a consequent reduction of the pyrolysis zone temperature and the rate at which volatiles are produced.
In many formulations of this type, both mechanisms are operative since the latter mechanism is of course also effective in reducing smoke. Typical additives having this latter effect are dawsonite, (NH.sub.4 or NaAl (OH).sub.2 CO.sub.3), hydrotalcite, Mg.sub.6 Al.sub.2 COOH).sub.16.4 H.sub.2), magnesium hydroxide and aluminum hydroxide, magnesium carbonate and magnesium borate.
Since typically these additives are less efficient than halogen containing additives, they are conventionally used in larger quantities, often up to 50 percent by weight of the polymer composition. For this reason, they are often considered as fillers, as well as smoke suppressants.
A real need exists in the field of compositions comprising styrene copolymers incorporating a halogen-containing component, which compositions begin to release halogen at temperatures above about 300.degree. C., for an effective smoke suppressant additive formulation that is effective in cooling the pyrolysis area so as to reduce smoke evolution while at the same time not inhibiting the effectiveness of the halogen in controlling vapor phase flame propagation mechanisms. This need is accented by the increasing number of laws and regulations relating to the combustion characteristics of polymers which go into items of household furniture , such as radios and television cabinets, tables, chairs, appliance housings, and other related uses and the increasing awareness that the presence of heavy smoke is significant contributory factor in many fires involving loss of life.
Typical S/MA copolymer compositions of reduced combustibility are described in co-pending application Ser. No. 644,491. The compositions disclosed therein comprise:
A. a rubber modified copolymer of a styrene monomer and maleic anhydride containing from 7 to 25 percent by weight of maleic anhydride based on the weight of the copolymer; wherein the rubber is selected from the group consisting of polyepichlorohydrin rubbers, polychloroprene rubbers, chlorinated polyethylene rubbers and ethylene vinyl acetate rubbers; wherein the amount of rubber used is in the range of from 1 to 30 percent by weight based on the weight of the styrene/maleic anhydride copolymers; with the proviso that when using an ethylene vinyl acetate rubber, the maleic anhydride content of the polymer is at least 10 percent by weight; PA0 B. from 7.5 to 15 percent by weight, based on the total weight of the composition, of an aromatic bromine compound which will provide at least 6 percent by weight of bromine to the composition; PA0 C. from 3 to 12 percent by weight, based on the total weight of the composition of a metal oxide selected from the group consisting of Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3, MoO.sub.3, SnO.sub.2 and WO.sub.3 ; PA0 D. from 3 to 25 percent by weight, based on the total weight of the composition of a smoke suppressant selected from the group consisting of dawsonite, magnesium carbonate, alumina trihydrate, calcium carbonate, magnesium borate and mixtures thereof. PA0 A. a styrene/maleic anhydride copolymer; PA0 B. a halogen-containing rubber; PA0 C. a halogenated aromatic flame retardant; PA0 D. a metal oxide flame retardant synergist; and PA0 E. a total of from 15 to 25 parts of a smoke suppressant combination of from 2 to 10 parts each of: PA0 wherein the number of parts of (ii) is at least one quarter the sum of the number of parts of (i), (ii) and (iii), all parts being by weight of the total polymeric composition. PA0 80/20-S/MA -- a copolymer of styrene with maleic anhydride in 80:20 proportions by weight. PA0 80/20-S/MA grafted onto Diene 55 -- A copolymerizable mixture of styrene and maleic anhydride polymerized in the presence of Diene 55 (see below) in given proportions. Amount of rubber is 10% of total monomer weight. PA0 80/20-S/MA grafted onto HYDRIN -- A copolymerizable mixture of styrene and maleic anhydride polymerized in the presence of a HYDRIN rubber (see below) in given proportions. Amount of rubber is 10% of total monomer weight. PA0 Hydrin -- an epichlorohydrin rubber supplied by B. F. Goodrich under that trade name. PA0 Triblock -- An experimental block copolymer rubber comprising 20 parts of styrene, 20 parts of butadiene and 60 parts of caprolactone supplied by Phillips Petroleum. PA0 Diene 55 -- A polybutadiene supplied by Firestone under that trade name. PA0 Dawsonite -- NaAl (OH).sub.2 CO.sub.3 PA0 Fr-300ba -- a flame retardant obtained commercially from Dow Chemical Company which is a brominated bisphenol ether.
Such polymers have flame-spread ratings of at least V-1 as determined by the UL-94 test and smoke evolution ratings of less than 450 Dm (flaming).
A way has now been found of reducing the smoke evolution of S/MA copolymers reinforced with a halogen-containing rubber, that is startlingly effective and dramatic in the degree of improvement obtained.